Design Guidelines for Military and Aerospace PCBs
Printed Circuit Boards are the heart of the electronics industry. They are used in a diverse range of applications starting from consumer electronics to complicated aerospace and military domains. The design considerations vary according to the requirements of the end product. Military and aerospace PCBs follow the most critical and strict design standards as they have to operate in tough environmental conditions with exposure to vibrations, contaminations, and extreme temperatures.
Military and aerospace PCB designs have to meet regulatory standards from the IPC trade association, military-specific standards by the US Department of Defense (DoD), and quality management standards for the aerospace industry set by organizations like NASA, DoD, and Federal Aviation Administration (FAA).
Based on the severity of the application, IPC has categorized electronic products into three classes. While the general consumer products come under Class 1, the dedicated service products are placed under Class 2. The military and aerospace PCBs fall under Class 3, which has a high-reliability requirement as detailed in IPC6012 Class 3/A.
Military and aerospace are the two sectors that demand superior performance and high precision PCB design. To ensure zero downtime, these PCBs adhere to the IPC-A-610E standard and go through quality checks at each stage starting from the design, PCB fabrication, assembly, and final deployment of the product. Several factors influence the development process of the military and aerospace PCBs.
Factors That Impact the Design and Assembly of Military and Aerospace PCBs
1. Extreme Temperature
Exposure to harsh temperatures (ranging between -150°C to +200°C) is one of the critical factors that affect aerospace and military PCB design.
PCB Material Selection:
Substrate material used in military-grade PCBs should withstand extreme temperatures. Depending on the operating temperature of the PCB, a substrate with suitable heat resistance (Tg) value has to be chosen. A minimum of 20°C margins have to be included. For example, if the required operating temperature is 160°C, then it is recommended to use a substrate with Tg=180°C.
High-temperature laminates with copper or aluminium substrates are suitable for these applications.
Materials like FR408, Pyralux AP, Nelco N7000-2HT all have high Tg values.
Coating and Surface Finish:
The military/aerospace PCBs are introduced to moisture and humidity along with the peak temperature. To protect the PCB components from these external factors, conformal coating of acrylic resin by spray method is recommended. The suggested surface finishes for these applications are electroless nickel with immersion gold coating (ENIG) or hot air solid levelling (HASL).
2. Shock and Vibration
In these applications, the operating environment can shake the PCB to a great extent. Avoiding counterfeit parts and mounting military-grade components is a standard followed to reduce the vibration effect on the PCB. Also, using the through-hole assembly technique can ensure strong adherence of the components and improve signal transmission.
To reduce the vibration effect, the circuit board design should avoid geometrical deformities like warping, twist, or bows. Designing a symmetrical stack up with uniform copper distribution is highly recommended in military and aerospace PCBs.
The military equipment designed should meet the MIL-STD-810 standard to be certified as field-ready.
3. Radiation Impact
One of the destructive forces impacting deployed aerospace and military electronic systems is radiation. This occurs naturally in space at higher altitudes and calls for radiation-hardened components to be used in the PCB design. Antifuse technology is used in aerospace PCB assembly to enhance the reliability and security of programmable devices. Military standards for rad-hard components are outlined in MIL-PRF-38535.
4. Radiofrequency Operation
Radiofrequency is used for communication in military and aerospace applications. The RF components used in military applications should meet the performance specifications described in the MIL-PRF-19500 standard.
A design compliant with the MIL-STD-461 standard will not suffer from EMI and also will not interrupt other surrounding devices.
Based on function and design, different types of PCBs can be used in military and aerospace applications. Rigid PCBs are used in aircraft cockpit instrumentation, axillary power units, control tower instrumentation, etc. Flexible and rigid-flexible PCBs are ideal for high shock and extended vibration environments. RF PCBs are used in communication circuits. Metalcore PCBs are preferred in both military and aerospace applications as they can enhance heat dissipation. HDI PCBs can reduce the size and weight of the device. They are mainly used in emergency response devices as they can provide quick signal transmission with reduced signal loss.
Design Guidelines for Military and Aerospace PCBs
- Only military-grade components should be used in the design, having a tolerance range of 1-2%.
- In the stack-up design, the dielectric thickness between two planes should be 3.5 mil minimum.
- The layout design should emphasize handling the maximum current load of the circuit. Using shorter traces, heavy copper (trace thickness between 3-4 ounces per square foot) and heat dissipation techniques are necessary for high-current circuit design.
- The annular ring is the copper pad area around the drilled hole and is used to provide a good connection with the copper trace and via. For military-grade PCB design, the defined width of the ring for the hole is 6-7 mils minimum for the external layer and internal layers. Recommended drill to copper clearance for military PCB is 7-8 mil.
- The aspect ratio is the board thickness to the diameter of drilled via. The recommended through-hole aspect ratio for military PCB is 10:1 and the maximum board thickness can be 100 mil.
- Guarding the clock signal is essential to generate a clean clock pulse. Using an aluminum enclosure as a physical shield can resolve the EMI issue.
- During component placement, excess heat-generating parts should be placed with a large clearance space and thermally sensitive components like regulators and converters should be isolated from heat-generating components.
- Press-fit components should be soldered to reduce vibrations.
Adhering to the performance and testing standards like MIL-PRF-50884, MIL-PRF-31032, MIL-PRF-55110, and MIL-PRF-19500 for military PCBs and AS9100, AS9100D for aerospace PCBs are mandatory.
Following these best practices will ensure a highly reliable and top-performing military and aerospace PCB design. Satellite equipment, electronics flight instrumentation systems, and central air data computers are some of the aerospace applications which are using the high end PCBs successfully. Unmanned vehicles, robotics, and defense navigation and communication systems are the latest military applications where PCBs are effectively deployed.
Military robotics is one of the fast-growing applications which is driving sophisticated and high-accuracy PCB design methods. Military robots are serving the armed forces in various forms like search and rescue robots, combat support, and ISR (intelligence, surveillance, and reconnaissance) applications. An experienced contract manufacturer can assist in building the best performing aerospace and military PCBs for these latest applications.
This article was written by Ken Ghadia, Sales Engineer, Technotronix (Anaheim, CA). For more information, visit here .